Hollow fan blade rib geometry

ABSTRACT

A fan blade has a main body extending between a leading edge and a trailing edge. Channels are formed into the main body from at least one open side. A plurality of ribs extend across the main body intermediate the channels. The fan blade has a dovetail and an airfoil extending radially outwardly from the dovetail. The ribs have a thickness defined as measured generally from the leading edge toward the trailing edge. A thickness of at least one of the ribs is generally thicker adjacent radially inner ends and becomes thinner moving in a radially outward direction.

BACKGROUND

This application relates to a hollow fan blade for a gas turbine engine, wherein a unique rib geometry is utilized.

Gas turbine engines may be provided with a fan for delivering air to a compressor section. From the compressor section, the air is compressed and delivered into a combustion section. The combustion section mixes fuel with the air and combusts the combination. Products of the combustion pass downstream over turbine rotors, which in turn are driven to rotate and rotate the compressor and fan.

The fan may include a rotor having a plurality of blades.

One type of fan blade is a hollow fan blade having a plurality of channels defined by intermediate ribs in a main fan blade body. An outer skin is attached over the main fan blade body to close off the cavities. The blades are subject to a number of challenges, including internal stresses that vary along a length of the fan blade.

SUMMARY

A fan blade has a main body extending between a leading edge and a trailing edge. Channels are formed into the main body from at least one open side. A plurality of ribs extend across the main body intermediate the channels. The fan blade has a dovetail and an airfoil extending radially outwardly from the dovetail. The ribs have a thickness defined as measured generally from the leading edge toward the trailing edge. A thickness of at least one of the ribs is generally thicker adjacent radially inner ends and becomes thinner moving in a radially outward direction.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described with regard to the specific and drawings, the following of which is a brief description.

FIG. 1A shows a fan blade.

FIG. 1B shows another feature of the FIG. 1A fan blade.

FIG. 2 is a cross-sectional view along line 2-2 as shown in FIG. 1A.

FIG. 3 shows a main body of the FIG. 1A fan blade.

FIG. 4 is a simplified view of one rib.

FIG. 5A is a first embodiment taken along line 5-5 of FIG. 4.

FIG. 5B is a second embodiment taken along line 5-5 of FIG. 4.

FIG. 5C is a third embodiment taken along line 5-5 of FIG. 4.

FIG. 6A is a first embodiment rib break-edge.

FIG. 6B is another embodiment rib break-edge.

FIG. 7 shows another area within the fan blade.

FIG. 8 shows a radially inner end of the channels.

DETAILED DESCRIPTION

A fan blade 20 is illustrated in FIG. 1A having an airfoil 18 extending radially outwardly from a dovetail 24. A leading edge 21 and a trailing edge 22 define the forward and rear limits of the airfoil 18.

As shown in FIG. 1B, a fan rotor 16 receives the dovetail 24 to mount the fan blade 20 with the airfoil 18 extending radially outwardly. As the rotor 16 is driven to rotate, it carries the fan blades 20 with it. There are higher stresses adjacent to the rotor 16, than occur radially outwardly of the rotor.

FIG. 2 shows a cross-section of the fan blade 20, at the airfoil 18. As shown, the leading edge 21 carries a cap 37 secured to a main body 28. A cover skin 32 closes off cavities or channels 30 in the main body 28. The main body 28, the cap 37 and the skin 32 may all be formed of various aluminum alloys. While aluminum alloys or aluminum may be utilized, other materials, such as titanium, titanium alloys, or other appropriate metals may be utilized.

As shown, a plurality of ribs 26 separate channels 30 in the cross-section illustrated in FIG. 2. These channels 30 are closed off by the skin 32. As shown, the channels 30 extend from an open end inwardly to a closed side. The open end is closed off by skin 32. It is within the scope of this invention, however, that the channel extends across the width of the main body 28, and there are two skins on opposed sides of the main body 28.

In addition, the channels may be filled with lighter weight filler material to provide stiffness, as known.

A contact area 132 at the forward face of the ribs 26 serves as a mount point for the skin 32, and receives an adhesive. Chamfers 38 are formed at the break-edges, or the edges of the ribs 26, and will be described in more detail below. As shown, the channels 30 have a side extent formed by a compound radius 34 and 36, again to be described in greater detail below.

FIG. 3 shows the main body 28. There are a plurality of channels 30 from the front or leading edge 21, to the back or trailing edge 22, and varying from the radially inner end toward the radially outer tip. As shown, some of the channels 30 extend generally radially upwardly. Other channels, such as channel 40, bend toward the leading edge 21. Other channels 41 simply extend generally from the middle of the main body 28 toward the leading edge 21.

To reduce the weight, it is desirable to maximize the amount of channels and minimize the amount of rib. However, there is also a need for additional stiffness adjacent the radially inner edge 42, to provide greater durability, and minimize blade pull. Thus, the ribs 26 may be formed such that they tend to be thicker adjacent a radially inner edge 42, and become thinner when moving toward the radially outer portions 44.

It is also desirable to form a blade which avoids certain operational modes across the engine operational range. Additional mass toward the tip or outer end of the blade raises challenges against tuning away from fundamental modes.

As shown schematically in FIG. 4, ribs 26 are thinner at radially outer end 44 than at the inner end 42. A thickness t₁ at the radially inner end 42 is greater than the thickness t₂ at the tip or radially outer end 44. In embodiments, a ratio of t₁ to t₂ may be between 1.1 and 8. As can be appreciated from FIG. 3, the variation need not be linear as shown in FIG. 4, and may be different across the several ribs.

As shown in FIG. 5A, a cross-section through the rib could be a trapezoid as shown in FIG. 5A, wherein the bottom 50, which extends into the main body 28, is larger than the outer end 48 which attaches to the skin 32. Sides 46 are angled between the two ends 48 and 50.

FIG. 5B shows a rectangular cross-section for the rib 26 wherein the ends 52 and 54 are generally of the same thickness, and the sides 56 are generally perpendicular to those ends.

FIG. 5C shows yet another embodiment, wherein the ends 58 and 60 are of different thicknesses, and the sides 62 curve relative to each other along a particular radius.

By modifying these several variables, a designer is able to tune or optimize the operation of the fan blade for its use in a gas turbine engine.

Notably, as will be explained below, it is desirable that the upper end 48/52/58 actually has a more complex surface at its break-edges.

FIG. 6A shows the actual break-edge 38 on a rib 26. The contact area 132 which will actually contact the skin, and provide a surface for receiving adhesive and securing the skin should be maximized. On the other hand, there are stresses which are induced at the break-edges, and thus a chamfer 38 is formed in this embodiment.

As shown in FIG. 6A, the rib 26 has a nominal thickness t₃ at the upper end, if not for the chamfers 38. Stated another way, t₃ is the distance between sides 200 at the end of the chambers 38. The chamfers 38 extend for a thickness c measured in a plane perpendicular to the top edge 132.

A ratio of c to t₃ may be between 0.02-0.15. The use of the chamfer at the break-edge location reduces the stress. There would otherwise be stress concentrations at that area. On the other hand, by utilizing a chamfer within the disclosed range, the amount of surface area available to provide a good adhesion to the cover is still adequate.

FIG. 6B shows an embodiment of a rib 64, wherein the break-edges are provided along a radius r₁. In embodiments, the ratio of r₁ to t₃ is between 0.02-0.15.

The features of the break-edges are disclosed in co-pending U.S. patent application Ser. No. ______, filed on even date herewith and entitled “FAN BLADE HAVING INTERNAL RIB BREAK-EDGE.”

FIG. 7 shows the surfaces 34 and 36 as illustrated in FIG. 2. The areas at that side of the channels 30 are prone to stress concentrations. A typical fillet, or single curve, may be considered for formation at that area to reduce stress. However, in the disclosed embodiment, a compound fillet having two curves 34 and 36 is utilized. Curve 34 is formed along a radius r₂ while curve 36 is formed along a radius r₃. As is clear, r₂ is greater than r₃. A ratio of r₃ to r₂ is between 0.03 and 0.25. More narrowly, it may be between 0.06 and 0.13. The use of the compound fillet provides a great reduction in stress concentration, which would otherwise be maximized at the general location of the curve 36.

An Application directed to the features of FIG. 7 has been filed as U.S. patent application Ser. No. ______, filed on even date herewith, and entitled “HOLLOW FAN BLADE CHANNEL CONFIGURATION TO REDUCE STRESS.”

Finally FIG. 8 shows a radially inner end, bottom or termination 100 of a channel 30. As shown, there is a compound curve or fillet including a bottom portion 104 formed at a radius r₄ and a side portion 102 formed at a radius r₅, which merges into the side of the ribs. As is clear, r₅ is greater than r₄. Again, this arrangement reduces a stress concentration at the corners which would otherwise be induced into the cavity terminations. In embodiments, a ratio of r₄ to r₅ is between 0.03 and 0.25.

An Application directed to the features of FIG. 8 has been filed as U.S. patent application Ser. No. ______, filed on even date herewith and entitled “FAN BLADE CHANNEL TERMINATION.”

The compound fillets as disclosed in FIGS. 7 and 8 reduce stress concentrations with minimum weight increase. Further, the compound fillets may be provided with minimal additional cost, because multi-pass machining is not required. Instead, a cutter with a compound radius shape may be utilized.

The fan blade as described above reduces stresses that are raised during operations, when mounted in a gas turbine engine.

Although embodiments have been disclosed, a worker of ordinary skill in the art would recognize the modifications which come within the scope of this Application. Thus, the following claims should be studied to determine the true scope and content. 

What is claimed is:
 1. A fan blade comprising: a main body extending between a leading edge and a trailing edge, and having channels formed into said main body from at least one open side with a plurality of ribs extending across the main body intermediate the channels, the fan blade having a dovetail, and an airfoil extending radially outwardly from said dovetail; and said ribs having a thickness defined as measured generally from said leading edge toward said trailing edge, with a thickness of at least one said ribs being formed to be generally thicker adjacent a radially inner end, and becoming thinner moving in a radially outward direction.
 2. The fan blade as set forth in claim 1, wherein a ratio of a thickness of said at least one rib selected near the radially inner end compared to a thickness selected near a radially outer end is between 1.1 and
 8. 3. The fan blade as set forth in claim 1, wherein a cross-section through at least one said rib from the open side, and toward the closed side is trapezoidal.
 4. The fan blade as set forth in claim 3, wherein an end of said trapezoidal rib spaced toward said open side is smaller than an end at said closed side.
 5. The fan blade as set forth in claim 1, wherein a cross-section taken through at least one rib moving from the open side and, toward said closed side is rectangular.
 6. A fan blade as set forth in claim 1, wherein a cross-section through at least one rib from the open side edge toward the closed side has generally curved sides, and is smaller at said open side than it is at said inner side.
 7. The fan blade as set forth in claim 1, wherein an outer skin closes off the channels at the at least one open side.
 8. The fan blade as set forth in claim 7, wherein said outer skin and said main body are both formed of aluminum or aluminum alloy.
 9. The fan blade as set forth in claim 7, wherein said outer skin and said main body are both formed of titanium or a titanium alloy.
 10. The fan blade as set forth in claim 7, wherein said channels extend from said open side to a closed side within said main body. 